30 AKT. '4. N. YATSU : 



come to line the arm-sinus, the spacious lumen thus formed 

 communicates directly with the body cavity. 



The 31. venlralis is seen as a few fibres imbedded among 

 the mesenchyme cells of the arm-sinus. It is probable that these 

 fibres have been formed out of mesenchyme cells (PI. IV., Fig. 

 69 m. vt.). 



3. Arm-apparatus. 



The arm-apparatus appears as a fold (arm-ridge) of the 

 ectoblast as has been stated in the foregoing section (PI. IV., 

 Figs. 55, 56, 57.). The ridge gradually increases in size, pro- 

 truding a great deal out of the mantle. The nuclei in the ridge 

 are spindle shaped and arranged in a single row (PI. IV., Fig. 

 57»). They are compact and their staining capacity is equal to 

 that of the nuclei in the stomodœum and exceeds that of the 

 nuclei in other parts. As mentioned above, the inner cavity of 

 the arm-apparatus (arm-sinus) is loosely filled with mesenchyme 

 cells (PI. IV., Figs. 65, 69). 



That the arm-apparatus is attached almost entirely to the 

 dorsal mantle is best shown in Fig. 58 (PL IV.) ; in later stages, 

 however, it gradually separates from the dorsal mantle. (PI. IV., 

 Figs. 61, 64.). 



On the dorsal side of the arm-apparatus, directly embracing 

 the oesophagus there is seen a very thin epithelium, whose nuclei 

 do not differ in any respect from those of the other ectoblast cells 

 (PI. IV., Figs. 61, 64.). This epithelium is, therefore, considered 

 not as a part of the original arm-ridge, but as derived from the 

 ectoblast dorsal to the ridge. 



